The present invention relates to a magneto-optical recording medium, and in particular relates to a magneto-optical recording medium adapted to the use of a magnetic field modulation recording method.
Methods for recording upon magneto-optical recording media can be broadly divided into optical modulation methods and magnetic field modulation recording methods. An optical modulation method is a method in which recording marks (magnetic domains) are formed by blinking a recording laser while applying a constant magnetic field to the medium. And a magnetic field modulation recording method is a method in which recording marks are formed upon the medium by varying a magnetic field while illuminating the medium with a recording laser at a constant power or with pulses of constant period and constant width. A magnetic field modulation recording method can be easily overwritten, and is suitable for mark edge recording. Here, mark edge recording means a method of recording in which channel bits are established in correspondence to the positions of the leading and trailing edges of the recording marks (magnetic domains) which are formed upon the magneto-optical recording medium. Position recording is an alternative to mark edge recording. Position recording is a recording method in which channel bits are established in correspondence to the positions of the centers of the recording marks. Mark edge recording can be applied for recording at a higher density than position recording, but more accurate recording mark control is required than with position recording. With magnetic field modulation recording methods accurate recording mark control is easy since the edges of the recording marks are determined almost entirely only by the timings of reversals of the magnetic field, and accordingly magnetic field modulation recording methods are suitable for mark edge recording. In contrast, with optical modulation methods, since the edges of the recording marks are determined by the on/off timing of the laser, accordingly the positions of the edges vary according to the temperature of the medium and according to the lengths of the recording marks (i.e., according to the amounts of heat accumulated in the recording marks). Thus, accurate recording mark control is difficult with optical modulation methods.
However, a magnetic head is required with a magnetic field modulation recording method, and moreover it is necessary for the inductance of the head to be minimized and for high currents to be switched, in order for the magnetic field to be reversed at high speed. A flying magnetic head such as disclosed in Japanese Patent Laying Open Publication Showa 63-217548 is advantageous in practice, so as to bring the magnetic head as close as possible to the recording layer of the recording medium, in order to ensure that the magnetic field which acts upon the recording layer is as great as possible.
In any case, with a magnetic field modulation recording method, it is desirable for the magneto-optical recording medium to have the characteristic of being able to be recorded with a low magnetic field, in order to utilize as low a magnetic field strength as possible. In order to reduce the magnetic field which is required for recording, it is disclosed upon page 53 of xe2x80x9cMagnetics Study Group Materials: MAG-86-95xe2x80x9d to optimize the amounts of heavy rare earth in a heavy rare earth xe2x80x94Fexe2x80x94Co three element composition recording layer. And upon page 268 of xe2x80x9cDIGEST of 11th Annual Conference on Magnetics in Japanxe2x80x9d it is described further to include a fourth element such as Nd, in order to enhance the characteristics in a low magnetic field. Japanese Patent Laying Open Publication Showa 62-128040 teaches to exchange-couple magnetic layers of different compositions so as to reduce the stray magnetic field, in order to improve the responsiveness to external magnetic fields. Further, in order to reduce the required intensity for the modulation magnetic field, Japanese Patent Laying Open Publication Showa 61-188758 discloses the concept of laminating together a layer with perpendicular magnetization and a layer with in-plane magnetization, so as efficiently to promote the magnetic flux in the perpendicular magnetization layer.
Even using these prior arts, it is difficult in practice to lower the required intensity of the modulation magnetic field below 100 Oersteds. Further, it may happen that the addition of the fourth element or of the additional layer with in-plane magnetization has the secondary effect of deteriorating the characteristics of the reproduced signals. In view of this problem, Japanese Patent Laying Open Publication Heisei 6-309711 teaches to provide, next to the recording layer of the magneto-optical recording medium, an ultra thin magnetic layer having a higher Curie temperature than that recording layer. The magnetic field required for modulation is considerably reduced by this layer construction. However, further reduction of the modulation magnetic field is desirable.
The objective of the present invention is to provide a magneto-optical recording medium suitable for a magnetic field modulation recording method, with which recording can be properly performed with a smaller modulation magnetic field than in the prior art.
The magneto-optical recording medium of the present invention comprises a recording layer and an auxiliary recording layer having a film thickness of less than or equal to 100 angstroms. This ultra-thin auxiliary recording layer enhances the magnetic characteristics of the recording layer and improves the magnetic field sensitivity of the medium. In other words, since the auxiliary recording layer enhances the magnetic characteristics of the recording layer, thereby it becomes possible to employ a material for the recording layer from which a film having an easy perpendicular axis of magnetization cannot easily be formed as a single layer because the value of its magnetization is great. Accordingly the magnetic field sensitivity of the medium is enhanced, and as a result it is possible to obtain a sufficient CN ratio even when recording is performed with a weak magnetic field. This type of medium is suitable for methods of recording using magnetic field modulation recording. Of course, it can also be applied to methods of recording using other modulation methods.
It is typically possible to use a heavy rare earth-transition metal alloy for the recording layer. Using xe2x80x9cHRExe2x80x9d has a term for xe2x80x9cheavy rare earth metalxe2x80x9d, if an alloy of composition given by HREx(Fe1xe2x88x92yCoy))1xe2x88x92x is used for the recording layer, it is necessary that 0.08xe2x89xa6xxe2x89xa60.20. Desirably, 0.08xe2x89xa6xxe2x89xa60.20 and also 0.0xe2x89xa6yxe2x89xa60.3. More desirably, 0.11xe2x89xa6xxe2x89xa60.16 and also 0.0xe2x89xa6yxe2x89xa60.3. Here, for example, it is possible to select one or a plurality of elements from Tb, Dy, and Gd for the heavy rare earth metal HRE.
The recording layer is not limited to being the above described heavy rare earth-transition metal alloy; other materials may be employed. For example, it would be acceptable to utilize a magnetic multilayer film of Pt/Co, Pd/Co, Pt/Fe, Au/Co, or Au/Fe; or an alloy like PtMnSb, CuCrSeBr, PtCo, or PdCo would also be acceptable; or, again, it is possible to utilize an oxide film like spinel ferrite or garnet. Whichever be the case, for the recording layer it is desirable to utilize a magnetic material of which the value of the magnetization at room temperature is at least 150 emu/cc.
It is necessary for the auxiliary recording layer to have a higher Curie temperature than that of the recording layer. Desirably, the Curie temperature of the auxiliary recording layer is at least 30K higher than that of the recording layer. It is typically possible to use a heavy rare earth-transition metal alloy for the auxiliary recording layer. If an alloy of composition given by HREv(Fe1xe2x88x92wCow)1xe2x88x92v is used for the auxiliary recording layer, it is desirable that 0.25xe2x89xa6vxe2x89xa60.35 and also 0.2xe2x89xa6wxe2x89xa61.0.
It is possible to use, not only a two layered structure of a recording layer and an auxiliary recording layer, but also a three layered structure of a stabilizing layer, a recording layer, and an auxiliary recording layer. The stabilizing layer has a thickness thinner than that of the recording layer, and it moreover has a Curie temperature lower than that of the recording layer and a coercivity at room temperature greater than that of the recording layer.
It is possible to prevent deterioration in the magnetic characteristics of the medium due to lapse of time by including Al, Ti, or Cr in the recording layer. For example, when A represents Al, Ti, or Cr and the composition of the recording layer is given by HREx(Fe1xe2x88x92yCoy)1xe2x88x92xxe2x88x92zAz, then it is desirable that 0xe2x89xa6zxe2x89xa60.12, and it is more desirable that 0.11xe2x89xa6xxe2x89xa60.16 and also 0.0xe2x89xa6zxe2x89xa60.08.
It is possible to increase the Kerr rotation in the short wavelength region by including Pr, Nd, or Sm in the recording layer. For example, when B represents Pr, Nd, or Sm and the composition of the recording layer is given by HREx(Fe1xe2x88x92yCoy))1xe2x88x92xxe2x88x92zBz, then it is desirable that 0xe2x89xa6zxe2x89xa60.12, and it is more desirable that 0.11xe2x89xa6xxe2x89xa60.16 and also 0.0xe2x89xa6zxe2x89xa60.08.
According to the theory of the present invention, it is also possible to apply a recording film of a two layer structure, made as a layered superposition of a recording layer and an auxiliary recording layer, or a recording film of a three layer structure, made as a layered superposition of a stabilizing layer, a recording layer and an auxiliary recording layer, as a magneto-optical recording medium which can be replayed using a Magnetically-Induced Super-Resolution (MSR) method. Various types of structure for magneto-optical recording mediums for use with MSR methods are known, such as for example ones having a replay layer additional to the recording film, and ones further having a control layer additional to the replay layer. With any of these types of magneto-optical recording medium, the magnetic field sensitivity of the medium is increased by using a recording film according to the present invention of a two layer structure or of a three layer structure. As a result, it becomes easy to perform recording at a high density suitable for replaying by a MSR method.